- Locate the application whose icon you'd like to change.
- Control-click on the application, pick Show Package Contents from the pop-up menu, and navigate to Contents » Resources.
- Locate the icns file in the Resources folder controlling the Dock icon. It will usually be called the application_name.icns
- Hold Option while dragging the file to your local directory and out of the application folder. Holding Option will make a copy instead of moving the file.
- Open the file up in Preview and save the file as a png instead of icns; this allows Photoshop to be able to open it.
- Usually the only thing that needs to be done to make an icon look great and fun is just a change in color scheme (here are some examples), so open up the file in Photoshop (I'm using Elements 4), and navigate to Enhance » Adjust Color » Adjust Hue/Saturation (Command-U).
- Ignore the saturation and lightness, unless of course you don't want to. Drag the hue slider to your prefered setting.
- Save the file.
- Download a program like img2icns to convert the file back to icns.
- Once converted, drag the old icon file out of the Resources folder of the application, and drag in the new. Re-open the app, and you will see the new and improved icon.
[robg adds: This is a fairly basic hint, but for those new to OS X who wish to tweak their icons, it's a pretty good starter tutorial. Note that you can avoid the file conversion steps if you use some of the available OS X icon editors. A good list of icon tools for OS X can be found here; Iconographer is a full-blown icon editor, and very good at what it does.]

